(a) Field of the Invention
The present invention relates to an optical system for an endoscope and, more particularly, to a single-lens reflex optical system for an endoscope.
(B) Description of the Prior Art
Most of conventional optical systems for endoscopes are of the twin-lens reflex type. That is, the observing optical system and photographing optical system are assembled in the distal end of the endoscope. Therefore, the distal end of the endoscope is necessarily long. This is not advantageous because pain of the patient who undergoes the examination becomes considerably large when such long distal end is inserted to the body cavity of the patient. To solve the above problem, a single-lens reflex optical system for an endoscope is provided as disclosed in U.S. Pat. No. 3,918,072 to Imai et al which bases priority on Japanese Patent Application No. 124,979/1972. An example of the above-mentioned single-lens reflex optical system for an endoscope is arranged as shown in FIG. 1. In FIG. 1, numeral 1 designates a cover glass, numeral 2 designates a prism, numeral 2' designates a correcting prism, numeral 3 designates a reflecting mirror having a small hole 3a at the center, numerals 4 and 5 respectively designate convex lenses, and numeral 6 designates a photographing lens. The cover glass 1, two biconvex lenses 4 and 5 and photographing lens 6 compose a photographing optical system which focuses an image on an object on a film surface 7. Numeral 8 designates a convex lens and numeral 9 designates an observing lens. Together with the cover glass 1 and convex lens 4, the convex lens 8 and observing lens 9 compose an observing optical system which focuses an image of the object on an end face 10a of an image guide 10. The afore-mentioned Japanese Patent Application also discloses another example of the single-lens reflex optical system for an endoscope which is arranged as shown in FIG. 2. The example shown in FIG. 2 differs from the example shown in FIG. 1 only in that a penta-prism 11 is used instead of the prism 2. The other details are substantially the same as those of the example shown in FIG. 1. (In FIG. 2, those members which are substantially the same as those shown in FIG. 1 are designated by the same numeral as those shown in FIG. 1.) Numeral 11' designates a correcting prism made of the glass of same quality as the penta-prism 11. Numeral 12 designates a mirror.
In cases of the above-mentioned known optical systems for endoscopes, the aperture stop of the photographing optical system and/or observing optical system is inclined in respect to the optical axes of respective optical systems. For instance, in case of the example shown in FIG. 1, the aperture stop (the small hole 3a serves as the aperture stop) of the photographing optical system is largely inclined in respect to its optical axis. In case of the example shown in FIG. 2, the aperture stop of the photographing optical system is only slightly inclined in respect to its optical axis. However, the aperture stop of the observing optical system is positioned on the surface of the mirror 12 which is formed on the penta-prism 11 and, therefore, the aperture stop is largely inclined in respect to the optical axis of the observing optical system as it is evident from FIG. 2. When the aperture stop is largely inclined in respect to the optical axis as described in the above, brightness of the focused image becomes uneven. This is because, when the aperture stop is not at a right angle to the optical axis but is inclined by an angle .theta. from the plane which is at a right angle to the optical axis, the aperture efficiency of the inclined aperture stop for the ray having a field angle .alpha. becomes as expressed by (cos.sup.2 (.theta. .+-. .alpha.)/cos.sup.2 .theta.). Therefore, for both of the photographing optical system and observing optical system, it is desirable that the aperture stop is positioned at an angle as far as possible close to a right angle in respect to the optical axis of the corresponding optical system.
Besides, for the photographing optical system it is possible to make the length from the lens system to the film surface shorter when the equivalent optical path of prisms constituting the photographing optical system is shorter. Therefore, for the optical systems to be used for endoscopes for which the diameter should be made as small as possible, it is desirable to make the equivalent optical path of prisms as short as possible. However, both of the above examples of known single-lens reflex optical system for an endoscope have a disadvantage that their equivalent optical paths are comparatively long.
Moreover, the distance from the entrance surface of the prism to the optical axis of lenses constituting the observing optical system and arranged between the prism and image guide should be made large. This is because the lens 1, which also serves as the cover glass, will interfere with the lens mount of the observing optical system when the above-mentioned distance is small and, consequently, it becomes difficult to arrange those members satisfactorily. To prevent such interference, in the example shown in FIG. 1, the cover glass 1 is arranged at a certain distance from the prism 2. However, this is not advantageous because the length from the cover glass to the film surface becomes large and it is contrary to the requirement to make the distal end compact. In case of the example shown in FIG. 2, the optical axis of the observing optical system is inclined toward the direction in which it goes away from the cover glass. However, this is not advantageous when manufacturing the endoscope because it is necessary to arrange the image guide in the inclined position. Besides, for single-lens reflex optical systems for endoscopes, it is preferable that the image obtained by the observing optical system becomes an erect image.